If we're:
- Generating the sitemap to specification
- Making it available at the correct URL(s)
- It is loading correctly
There's not really much more we can do.
Ironically, at one point, Google stopped processing sitemaps that were hosted on a sitemap.php URL hence why we started recommending (and made available) sitemap.xml.
If sitemap.php is working for you then continue using it.
But ultimately this strange and inconsistent behaviour is really a question that Google themselves ought to answer.